1. Field of the Invention
This invention relates to a digital-to-analog (D/A) converter for converting digital signals, such as audio or video digital signals, into analog signals.
2. Description of the Related Art
There has recently been known a 1-bit D/A converter for translating digital audio or video signals into original analog signals, such as 1-bit D/A converter, as disclosed in e.g. US Pat. Nos. 5,021,788 with 5,148,168. The 1-bit D/A converter has many advantages not achieved with a multi-bit type converter particularly with respect to linearity, such as freedom from zero-crossing distortion.
An output of the 1-bit D/A converter is a pulse train taking on two voltage values of 0 V and a source voltage, usually on the order of 5 V. The output level is determined by the product of the pulse density and the source voltage. Specifically, an analog output waveform is obtained by passing an output of the D/A converter through a low-pass filter (LPF).
Meanwhile, the power source voltage, which is the wave crest value of an output pulse of the 1-bit D/A converter, is the external power source itself and is subject to disturbances. If the power source voltage fluctuates or the noise is superimposed thereon, the disturbed voltage is directly presented via LPF in the ultimate analog output.
Besides, if the 1-bit D/A converter is arranged as an integrated circuit, the power source voltage is that of the integrated circuit (IC), which cannot be raised to a value higher than the standard value of 5 V if a satisfactory IC operation is to be maintained. The result is that a limitation is necessarily placed on the output signal level which is usually set to the level on the order of 1 V or less on an average. This lower voltage is amplified by providing a gain in an analog LPF unit downstream of the D/A converter for increasing the signal level. However, this leads to amplification of the noise as well.